Most data processing systems include both volatile and nonvolatile memory devices. In general, volatile memory devices, such as random access memory, provide faster access times than nonvolatile memory devices, such as magnetic or optical disks. However, nonvolatile memory is generally less expensive and less susceptible to data loss.
To take advantage of the persistent nature of nonvolatile memory, an object, such as a data item in a database system, is typically stored on nonvolatile memory (i.e. database) until the object is required by a process. To take advantage of the speed of volatile memory, a copy of the object is loaded into volatile memory when the object is required by a process. Once the object is loaded into volatile memory, the process can quickly access and make changes to the copy of the object. At some later point in time, the copy of the updated object is written back to the database in order to reflect the changes that were made by the process.
For example, in a database system, a section of volatile memory known as a buffer cache is generally used by the processes for accessing and manipulating information contained within the database. In order for a process to access or change data that is stored in the database, a copy of the data is first loaded from the database into the buffer cache. After the data is loaded in the buffer cache, the process can then quickly access and manipulate the copied data version. At some later point in time, the contents of the buffer cache are written back to the database in order to reflect any changes that were previously made to the copied data version.
Typically, the buffer cache includes multiple buffers that are shared among one or more processes that are executing on a database server. When a process executes a transaction that requires a change to an item within a data block, a copy of the data item is loaded into a buffer in the buffer cache. Any changes are then made to the data within the buffer.
Because of the nature of volatile memory, various types of failures can cause the information contained within the buffers to be lost. If the volatile memory contains updated copies of data items, the changes may be lost if a failure occurs before the changes are written back into the database. In many applications, such loss of information is unacceptable.
Therefore, recovery techniques have been developed to reduce the possible loss of information due to failures within a database system. According to one approach, data is made "recoverable" whenever it becomes critical for the data to survive a failure. Data is considered to be "recoverable" when enough information to reconstruct the data after a failure is stored in nonvolatile memory. For example, in database systems it is considered critical that the changes made by a particular committed transaction be reflected on the database and the changes made by a particular aborted transaction be removed from the database.